Gamma (“γ”)-aminobutyric acid (“GABA”) is one of the major inhibitory transmitters in the central nervous system of mammals. GABA is not transported efficiently into the brain from the bloodstream (i.e., GABA does not effectively cross the blood-brain barrier). Consequently, brain cells provide virtually all of the GABA found in the brain (GABA is biosynthesized by decarboxylation of glutamic acid with pyridoxal phosphate).
GABA regulates neuronal excitability through binding to specific membrane proteins (i.e., GABAA receptors), which results in opening of an ion channel. The entry of chloride ion through the ion channel leads to hyperpolarization of the recipient cell, which consequently prevents transmission of nerve impulses to other cells. Low levels of GABA have been observed in individuals suffering from epileptic seizures, motion disorders (e.g., multiple sclerosis, action tremors, tardive dyskinesia), panic, anxiety, depression, alcoholism and manic behavior.
The implication of low GABA levels in a number of common disease states and/or common medical disorders has stimulated intensive interest in preparing GABA analogs, which have superior pharmaceutical properties in comparison to GABA (e.g., the ability to cross the blood brain barrier). Accordingly, a number of GABA analogs, with considerable pharmaceutical activity have been synthesized in the art (See, e.g., Satzinger et al., U.S. Pat. No. 4,024,175; Silverman et al., U.S. Pat. No. 5,563,175; Horwell et al., U.S. Pat. No. 6,020,370; Silverman et al., U.S. Pat. No. 6,028,214; Horwell et al., U.S. Pat. No. 6,103,932; Silverman et al., U.S. Pat. No. 6,117,906; Silverman, International Publication No. WO 92/09560; Silverman et al., International Publication No. WO 93/23383; Horwell et al., International Publication No. WO 97/29101, Horwell et al., International Publication No. WO 97/33858; Horwell et al., International Publication No. WO 97/33859; Bryans et al., International Publication No. WO 98/17627; Guglietta et al., International Publication No. WO 99/08671; Bryans et al., International Publication No. WO 99/21824; Bryans et al., International Publication No. WO 99/31057; Belliotti et al., International Publication No. WO 99/31074; Bryans et al., International Publication No. WO 99/31075; Bryans et al., International Publication No. WO 99/61424; Bryans et al., International Publication No. WO 00/15611; Bellioti et al., International Publication No. WO 00/31020; Bryans et al., International Publication No. WO 00/50027; and Bryans et al., International Publication No. WO 02/00209).
Pharmaceutically important GABA analogs include, for example, gabapentin (1), pregabalin (2), vigabatrin (3), and baclofen (4) shown above. Gabapentin is a lipophilic GABA analog that can pass through the blood-brain barrier, which has been used to clinically treat epilepsy since 1994. Gabapentin also has potentially useful therapeutic effects in chronic pain states (e.g., neuropathic pain, muscular and skeletal pain), psychiatric disorders (e.g., panic, anxiety, depression, alcoholism and manic behavior), movement disorders (e.g., multiple sclerosis, action tremors, tardive dyskinesia), etc. (Magnus, Epilepsia, 1999, 40:S66–S72). Currently, gabapentin is also used in the clinical management of neuropathic pain. Pregabalin, which possesses greater potency in pre-clinical models of pain and epilepsy than gabapentin is presently in Phase III clinical trials.
A significant problem with many GABA analogs is intramolecular reaction of the γ amino group with the carboxyl functionality to form the γ-lactam, as exemplified for gabapentin below. Formation of γ-lactam (5) presents serious difficulties in formulating gabapentin because of its toxicity. For example, gabapentin has a toxicity (LD50, mouse) of more than 8000 mg/kg, while the corresponding lactam (5) has a toxicity (LD50, mouse) of 300 mg/kg. Consequently, formation of side products such as lactams during synthesis of GABA analogs and/or formulation and/or storage of GABA analogs or compositions of GABA analogs must
be minimized for safety reasons (particularly, in the case of gabapentin).
The problem of lactam contamination of GABA analogs, particularly in the case of gabapentin, has been partially overcome through use of special additional purification steps, precise choice of adjuvant materials in pharmaceutical compositions and careful control procedures (Augurt et al., U.S. Pat. No. 6,054,482). However, attempts to prevent lactam contamination have not been entirely successful, in either synthesis or storage of GABA analogs such as gabapentin or compositions thereof.
Rapid systemic clearance is another significant problem with many GABA analogs including gabapentin, which consequently require frequent dosing to maintain a therapeutic or prophylactic concentration in the systemic circulation (Bryans et al., Med. Res. Rev., 1999, 19, 149–177). For example, dosing regimens of 300–600 mg doses of gabapentin administered three times per day are typically used for anticonvulsive therapy. Higher doses (1800–3600 mg/d in divided doses) are typically used for the treatment of neuropathic pain states.
Sustained released formulations are a conventional solution to the problem of rapid systemic clearance, as is well known to those of skill in the art (See, e.g., “Remington's Pharmaceutical Sciences,” Philadelphia College of Pharmacy and Science, 17th Edition, 1985). Osmotic delivery systems are also recognized methods for sustained drug delivery (See, e.g., Verma et al., Drug Dev. Ind. Pharm., 2000, 26:695–708). Many GABA analogs, including gabapentin and pregabalin, are not absorbed via the large intestine. Rather, these compounds are typically absorbed in the small intestine by the large neutral amino acid transporter (“LNAA”) (Jezyk et al, Pharm. Res., 1999, 16, 519–526). The rapid passage of conventional dosage forms through the proximal absorptive region of the gastrointestinal tract has prevented the successful application of sustained release technologies to many GABA analogs.
Thus, there is a significant need for effective sustained release versions of GABA analogs to minimize increased dosing frequency due to rapid systemic clearance of these compounds. There is also a need for pure GABA analogs, (particularly gabapentin and pregablin analogs) which are substantially pure and do not spontaneously lactamize during either formulation or storage.